Kinetics of non-isothermal crystallization of poly (ethylene terephthalate) (PET) with different cyclic oligomer contents was studied by means of differential scanning calorimetry. The maximum value of crystallization rate constant, Kmax, which was expressed as functions of half width of crystallization rate constant curve, cooling rate, and Avrami exponent was used to analyze the crystallization behavior. It was found that the crystallization rate of PET was decreased by the presence of cyclic oligomer.